What a ridiculous assumption.
Maybe they consider themselves and their partner to be equal halves of a whole. You know, the definition of half.
As a non native speaker you'll probably just feel upset/hopeless/angry.
From my experience, "non-native" here includes people who are "fluent".
So we arrive at the situation where my OH-SO beloved wife is fluent in English and is definitely better than me at writing clearly constructed English essays but when it comes to usage of random idioms/slang or understanding local (and foreign!) English accents I have a very clear advantage.
I wonder how acronyms such as OTOH even become so well known that they can be used without fear or not being understood? When is that threshold reached? Is using OH now the beginning of a new well-known acronym? I guess only time will tell...
And to answer the question - the threshold is when people stop complaining about the use :)
It also isn't an exact synonym of "conversely".
I've been an extensive internet user for decades and I don't have it in memory, so I'm not sure how to feel about your assertion. I'm not the only person saying this.
I'm sure that depends on the tolerance. "assist" and "help"? "dog" and "canine"? "purchase" and "buy"?